1. Field of Invention Technology
The present invention relates to an apparatus for evaluating a crack on an object by image processing of a digital image and, in particular, to an apparatus for evaluating the width of a crack on the surface of a concrete structure by image processing of a digital still color image.
2. Description of Related Art
On the surface of a concrete structure sometimes appears damage informing indices such as a crack, scaling, emergence of free lime (hereinafter referred to as free lime), and a sand pocket caused by structural or material deterioration due to aging. The damage degree of the concrete structure is evaluated based on the emergence conditions of the damage informing indices. The evaluator has located and evaluated the emergence conditions of the damage informing indices (the crack direction, length, width, and density; and the areal size of scaling and free lime) on the ground or through a silver-halide film photographic image of the concrete structure to be evaluated. The evaluator, however, necessarily have to be considerably trained and experienced to evaluate properly.
Measurement and digitalization of the damage informing indices support objective evaluation. The crack length and width, the area of scaling and free lime region, etc. are subjected to the digitalization.
The digitalization has been performed by, on the ground, directly measuring the damaged portion subject to the digitalization with a scale or a clearance gauge or by comparing the portion with standard photographs of cracks with various widths; or by measuring the damage informing indices on a silver-halide photographic image with a scale or a planimeter.
However, the activities on the ground require that the evaluator should all the way come to the field and come near the damaged portion, resulting in much time and trouble to perform the evaluation. On the other hand, when measuring the damage informing indices on a silver-halide photographic image with a scale or a planimeter, because, starting with taking photographs on the ground, developing and printing time is necessary to obtain a photographic image and because, furthermore, much trouble must be taken to measure the indices on the obtained photographic image, it has been difficult to quickly complete the evaluation process.
With regard to a number of concrete structures built in the decade began in 1965, damaged portions on them have recently come under observation, and the need to evaluate damage on concrete structures is keenly felt. But, because, as mentioned above, the number of the evaluator is limited and the evaluation requires a lot of time, the increasing demand for the damage evaluation might not be satisfactorily satisfied.
Recently, in consideration of the above situation, an image processing apparatus that digitizes the crack length, the area of scaling and free lime region, etc. through image processing based on a digital still image taken with a digital still camera has gradually been employed. A digital still image taken with a digital still camera facilitates quick acquisition of necessary images, and the digitizing measurement of the crack length, the area of scaling and free lime region, etc. can be quickly completed by tracing the crack and the boundaries of scaling and free lime region displayed on the display portion of the image processing apparatus by the use of a pointer such as a mouse.
The image processing of the image processing apparatus is implemented multiplying the number of pixels lying on a line defined with the pointer on the digital still image or the number of pixels lying within a closed curve drawn with the pointer on the digital still image by the dimension or area on a target object corresponding to the pixel size, i.e., the actual size. The xe2x80x9cpixelxe2x80x9d is to be understood to be a xe2x80x9cpicture elementxe2x80x9d which constitutes a digital still image. A picture element constituting the image pick-up device of a digital camera is also called a pixel. Each pixel of a digital camera generally corresponds to a pixel of an image.
In the above-mentioned apparatus, a binalizing process with a threshold value of an appropriate density value and edge enhancing process are appropriately performed.
In the meantime, a conventional image processing apparatus is adapted to monochrome images taken with, for example, an industrial digital camera. The line width measurement method used in the conventional image processing apparatus presupposes that a measured line width corresponds to the width from a number of pixels of an image pick-up device, and there has been no measurement method applicable to a narrow line width corresponding to a size less than a single pixel size.
Actually, in many cases, the aforementioned crack width is narrow. When the crack width corresponds to a size less than the single pixel size of an image pick-up device of a digital camera, the conventional image processing apparatus, even if it could visually show the existence of different cracks to be measured, was not able to detect the crack width difference among the cracks.
To address such narrow crack width, a high shooting magnification is required in order that the minimum crack width to be measured may correspond to the size from a plurality of pixels on the image pick-up device. This reduces the target object area taken in on a single image, and thus a number of segmented photographic images need to be taken to cover the overall target object; consequently, the longer time is necessary to complete the shooting and the analysis of the images, and much trouble has to be taken to control the images. Furthermore, it is not easy to determine, with regard to each image, the corresponding part on the target object, and, as a result, it is not easy to get the crack in perspective only by the images.
The actual size corresponding to one pixel of an image pick-up device is defined as the product of the pixel size of the image pick-up device and a inversion of a shooting magnification. Digital cameras with small pixel size commercially available now have a pixel of, for example, 0.004- or 0.01-mm-square. The shooting magnification is defined as the ratio of the actual size photographed to the image pick-up device size. The image pick-up device sizes of the above-mentioned digital cameras are 6.4*4.8 mm and 23.7*15.6 mm, respectively.
Color digital cameras with high pixel density have recently been developed, and the actual sizes corresponding to one pixel of their image pick-up devices are almost equal to those of industrial digital cameras. To evaluate the damage degree of concrete, it is also important to observe discoloration conditions on and around a crack, scaling, and a free lime portion. Thus, it is preferable that a photographic image carrying the damaged concrete portion""s image be a digital color image. Additionally, the techniques used in the above-mentioned conventional image processing apparatus are utilized also in the damage evaluation through such a digital color image; however, the narrow crack width measurement problem is still pending.
It is an object of the present invention to provide a crack evaluation apparatus that can numerically evaluate a crack width less than a single pixel size of a digital image through processing the digital image.
It is another object of the present invention to provide a crack evaluation apparatus that can evaluate a crack width accurately.
It is another object of the present invention to provide a crack evaluation apparatus that can evaluate a crack width by processing an image including noises.
In order to achieve the above objects the present invention provides an image processor for a digital image of an object having a crack comprising a counter for counting a number of pixels of the image relating to the crack within a selected portion of the image, the selected portion being of a known area, an investigator for investigating the relationship among the pixels relating to the crack within the selected portion of the image, and a width evaluator for evaluating the width of the crack corresponding to the selected portion of the image on the basis of the number of the pixels and the relationship among the pixels. Thus, the processor can evaluate the width of the crack which would be impossible in the prior art.
Pursuant to another aspect of the present invention, the image processor further comprises a noise eliminator for eliminating pixels not relating to the crack from the selected portion of the image so that the pixels not relating to the crack is not counted by the counter. Thus, the processor can evaluate the width of the crack accurately against the noises of the image.
Pursuant to still another aspect of the present invention, the digital image which is processed by the image processor is a color digital image. Thus, the width of the crack less than the size of the pixel of the image can be evaluated.
Pursuant to still another aspect of the present invention, the image processor further comprises a compensator for compensating a distortion of the digital image to get an orthogonal projection image, wherein the selected portion of the image is set on the orthogonal projection image. Accordingly, the processor can evaluate the width of the crack accurately against the distortion of the image. A plurality of thus compensated images can be combined to a form a single image which covers wide area. This makes it possible for the processor to evaluate a large floor board easily.
According to the above features and advantages of the present invention, a narrow crack width can be evaluated with low magnification photography of a wide angle, which makes it easy to identify the photographed area. Further, a less number of shots can cover an entire target field, which reduces the time for taking the photographies and analizing them.